Dispersion of aluminium oxide

ABSTRACT

A dispersion containing a high-surface-area, pyrogenically-produced aluminium oxide with a surface area and BET of more than 115 m 2 /g and a Sears number of more than 8 ml/2 g, is produced by mixing a high-surface-area, pyrogenically-produced aluminium oxide having a BET specific surface area of more than 115 m 2 /g and a Sears number of more than 8 ml/2 g, with water, setting a pH value of 2 to 11 (preferably 3 to 8) and dispersing the mixture by the introduction of controlled shearing forces.  
     It can be used for the production of Inkjet media, digital imaging products and other printing medi

[0001] The invention relates to a dispersion of pyrogenically-produced aluminium oxide, a process for its production and its use.

[0002] The invention provides a dispersion, which is characterised in that it contains a high-surface-area, pyrogenically-produced aluminium oxide, which has a BET specific surface area of more than 115 m²/g and a Sears number of more than 8 ml/2 g.

[0003] In one embodiment of the invention, the aluminium oxide can be chracterised in that the dibutylphthalate absorption of the powder, measured on a 16 g portion, is not measurable (no end-point detection).

[0004] The pyrogenically-produced aluminium oxide that can be used according to the invention, can be produced by the flame oxidation, or preferably flame hydrolysis, method, an evaporable aluminium compound, preferably the chloride, being used as the starting material.

[0005] The content of aluminium oxide can be 25±15 wt. %.

[0006] The dispersion may contain the following additional constituents:

[0007] Organic or inorganic acids or bases, salts, additives such as ionic or non-ionic surfactants, polymers, polyelectrolytes or biocides as well as other auxiliary substances to stabilise the dispersion or to increase its quality or storage stability.

[0008] The dispersion according to the invention may also contain, in addition to aluminium oxide, inorganic acids (such as phosphoric acid, nitric acid, sulfuric acid, . . . ), organic acids (such as formic acid, acetic acid, propionic acid, oxalic acid, malonic acid, . . . ), inorganic or organic bases (such as potassium hydroxide, organic amines, . . . ), salts, (such as sodium chloride, potassium formate, calcium nitrate), buffer systems (such as for example potassium dihydrogenphosphate/phosphoric acid buffer, acetic acid/sodium acetate buffer, . . . ) ionic, or non-ionic surfactants, polyelectrolytes, polymers or other dispersing agents or stabilisers, and also biocides.

[0009] The invention further provides a process for the production of the dispersion according to the invention, which is characterised in that a high-surface-area, pyrogenically-produced aluminium oxide, which has a BET specific surface area of more than 115 m²/g and a Sears number of more than 8 ml/2 g, is mixed with water, a pH value of 2 to 11, preferably 3 to 8, in particular 2 to 8, is set and the mixture is dispersed by the introduction of shearing forces.

[0010] The invention is formed by mixing a high-surface-area, pyrogenically-produced aluminium oxide, which has a BET specific surface area of more than 115 m²/g and a Sears number of more than 8 ml/2 g, with water and additives in accordance with claim 4, setting a pH value of 2 to 11 (preferably 3 to 8), and dispersing the mixture by the introduction of controlled shearing forces. To disperse the high-surface-area, pyrogenically-produced aluminium oxide, shearing equipment such as rotor-stator-type machines (batch- or continuous in-line machines), ball mills, pearl mills, agitated ball mills or high-energy shearing processes (in which the dispersion is dispersed under high pressure at pressures of over 1000 bar) is used and the dispersions obtained have a mean particle size distribution of d₅₀<150 nm (d₅₀ less than 150 nm; measured by dynamic light scattering).

[0011] The dispersion according to the invention has the following advantages:

[0012] high cationic charge on the surface of the particles, as compared with dispersions produced from aluminium oxides (-oxide hydroxides) by the wet chemical method

[0013] particle size distributions of the dispersion can be set in a defined way, in contrast to dispersions of other pyrogenic aluminium oxides or aluminium oxides (-oxide hydroxides) produced by the wet chemical method, which are produced by different dispersion processes

[0014] high dispersion purity, as compared with dispersions produced with aluminium oxides (-oxide hydroxides) produced by the wet chemical method

[0015] precisely-controllable electrolyte level within the dispersion, as compared with dispersions of other pyrogenic aluminium oxides or aluminium oxides (-oxide hydroxides) produced by the wet chemical method, the electrolyte concentration of which cannot be set precisely as a result of the starting powder or the dispersion process

[0016] very high surface area of the pyrogenic aluminium oxide as compared with dispersions of other pyrogenic aluminium oxides or aluminium oxides (-oxide hydroxides) produced by the wet chemical method

[0017] highly distinctive “structure/crosslinking” of the aluminium oxide primary particles within the dispersion, as compared with dispersions of other pyrogenic aluminium oxides or aluminium oxides (-oxide hydroxides) produced by the wet chemical method

[0018] a high degree of hardness and abrasion resistance of the particles distributed in the dispersion, as compared with dispersions of aluminium oxides (-oxide hydroxides) produced by the wet chemical method, in which the primary particles are less hard.

[0019] The dispersion according to the invention is eminently suitable for further processing in Inkjet coating pigments. The invention further relates to a process for the production of Inkjet coating pigments that have been produced with the dispersions according to the invention and the glossy media produced from them. These media are characterised by a very high gloss value, very high colour densities when printing, large colour space, extremely low drying times, a high absorption capacity and (in comparison with other aluminium oxides) increased water resistance, all of which are desirable characteristics.

[0020] The following examples demonstrate that the dispersion according to the invention is superior to known dispersions with regard to its processability in a coating pigment and in the associated Inkjet medium.

[0021] The recipes used for Inkjet coating pigments are guide recipes. Thus binders other than polyvinylalcohol (with various saponification grades) can be used in combination or alone, such as for example polyvinylpyrrolidone, polyvinylacetate, silanised polyvinylalcohols, styrene-acrylate lattices, styrene-butadiene lattices, melamine resins, ethylene-vinylacetate copolymers, vinylpyrrolidone-dimethylaminopropylenemethacrylamide copolymers, polyurethane resins, synthetic resins such as polymethylmethacrylates, polyester resins (for example unsaturated polyester resins), polyacrylates, modified starch, casein, gelatines, cellulose derivatives (for example carboxymethylcellulose) and also in other quantity ratios to the pyrogenic aluminium oxide than those stated in the application example.

[0022] The coating according to the invention may also contain, in addition to the dispersion according to the invention, one or more other pigments, and/or dispersions. These may be: calcium carbonates, layered silicates, aluminium silicates, plastic pigments (for example polystyrene, polyethylene, polypropylene), silicas (for example colloidal silicas, precipitated silicas, silica gels, pyrogenic silicas, pyrogenic mixed oxides), cationised variants of the stated silica compounds, aluminium compounds (for example aluminium sols, colloidal aluminium oxides and their hydroxy compounds, such as pseudo-boehmite, boehmite, aluminium hydroxide), magnesium oxide, zinc oxide, zirconium oxide, magnesium carbonate, kaolin, clay, talc, calcium sulfate, zinc carbonate, satin white, lithopones, zeolites or dispersions of these compounds.

[0023] Both the solid content of the dispersion and also that of the coating pigment can be varied in comparison with the application examples. Further possible variations consist in changing the pH value, the dispersion method, the composition of the coating pigment formulation and the dispersion medium.

[0024] To increase the water-resistance of the binder system and thus the coating, crosslinkers may be used, such as: zirconium oxides, boric acid, melamine resins, glyoxal and isocyanates and other molecules that bind the molecule chains of the binder system with each other.

[0025] Furthermore, auxiliary agents such as optical brighteners, de-foaming agents, wetting agents, pH buffers, UV absorbers and viscosity improvers can also be used.

[0026] Examples of cationic substances for the cationising of the binder are: polyethyleneimine, polyallylamines, polyvinylamines, polymers consisting of cationic monomers such as primary, secondary and tertiary amines, which are located on the side-chains, and quarternary ammonium compounds.

[0027] The coating structure of the medium may also differ from the application examples, in that the coating may consist of several coating layers of the same type or other layers, and the coating that has been produced with the dispersion according to the invention is only located in one of the layers present. Thus, for example, further ink-absorptive coatings (for example precipitated silica coatings) may be applied under the Inkjet coating according to the invention. Furthermore, one or more polymer layers (e.g. polyethylene) can be applied to the substrate and/or to the coating according to the invention, in order to increase the mechanical stability and/or gloss of the coating (for example photographic base paper, lamination). Even if the Inkjet medium contains one or more of the layers described above, and is provided with the coating according to the invention, printability is improved for the majority of printing applications.

[0028] The coating pigment recipe can be applied to a substrate by all conventional application processes (alone or in combination). Conventional processes (which are not, however, restricted to the stated processes) are: rolling, blade application, airbrushing, doctor blade (profiled, smooth, split), the cast-coating process, film pressing, bonding-pressing, curtain-coating and slot-die application (for example coating blade). The processes allowing extremely homogeneous coating, for example cast-coating, curtain-coating and slot-die application, are preferred.

[0029] The coated substrate can be dried by all of the conventional processes (alone or in combination). Conventional processes (which, however, are not restricted to the stated processes) are: air- or convection drying (for example hot air channel), contact- or conduction drying, energy radiation drying (for example infra-red and microwave).

[0030] The invention further provides coatings, which have an aluminium oxide-binder weight ratio of 8:1 to 1:2. The coatings can be used for the production of Inkjet-media, digital imaging products and other printing media.

[0031] The invention further provides recording media consisting of aluminium oxide dispersions, which are characterised in that they consist of substrates with glossy coatings, in which the coatings contain VP Alu 130 dispersions and dispersion additives (such as acids, bases, surfactants, polymers, polyelectrolytes etc.) and a binder system, the aluminium oxide particles that build up the coating originating from a dispersion according to the invention, with an average particle size of d₅₀<200 nm (d₅₀ less than 200 nm; measured by dynamic light scattering).

[0032] The aluminium oxide dispersion can be used in coatings for materials such as paper, metals, glass, ceramics, plastics, polymers, rubbers and textiles. Furthermore, it can be used to polish or clean metals, semi-conductor elements in the electronics industry, glass, ceramics and other hard materials.

[0033] The coatings that have been produced from coating pigments, may have a solid matter content of 15%, and preferably of 18% and more.

[0034] The coatings may have a substrate with a substance weight of 80 to 250 g/m². The substrate may have a Cobb₁₀value of 0 to 60.

[0035] A water-impermeable sealing coat may be applied to the substrate. The substrate may be a polymer film.

[0036] The coatings may be used for the production of Inkjet media (such as paper, films, textiles, . . . ), digital imaging products and other printing media.

[0037] The dispersions according to the invention may be used for the coating of fluorescent tubes, lightbulbs or other light sources.

EXAMPLE 1 Production of the Aluminium Oxide that can be used According to the Invention

[0038] 320 kg/h previously-evaporated aluminium trichloride (AlCl₃) is burned together with 100 Nm³/h hydrogen and 450 Nm³/h air in a burner of known construction.

[0039] After the flame reaction, the fine-particle, high-surface-area aluminium oxide is separated from the hydrochloric acid gases that have also formed, in a filter or cyclone, any HCl traces still adhering being then removed by treatment with moistened air at increased temperature.

[0040] The high-surface-area pyrogenic aluminium oxide formed has the physical/chemical reference data shown in Table 1. The data for the pyrogenic aluminium oxide that can be obtained commercially from Degussa-Hüls AG/Frankfurt (commercial name Aluminiumoxid C) are also included in Table 1 for reference. TABLE 1 High-surface-area aluminium oxide according to the invention Aluminium Unit Alu 130 oxide C BET m²/g 121 100 Specific Surface area Sears number ml/2 g 9.38 7.05 (pH 4 to 9) PH 4% aqueous 4.93 4.5 dispersion Drying loss wt. % 3.3 3.0 Bulk density g/l 55 48 Tamped density g/l 63 57 DBP-Absorption wt. % Not measurable, 231 no detectable end point.

[0041] The measurement of the Sears number is disclosed in EP 0 717 008.

EXAMPLE 2 Production of the Dispersion According to the Invention

[0042] First, 280 litres de-ionised water are brought to pH 3.9 with a propionic acid in a receiving vessel. 80 kg of the powder described in example 1 with a surface area greater than 115 m2/ g (equivalent to 20 wt.-% aluminium oxide) are then introduced into the water with a rotor-stator machine. After incorporating the whole quantity of the powder, the suspension obtained is intensively sheared for ca. 60 minutes.

[0043] Whilst the powder is being introduced, the pH value is maintained at pH=4.0 to 4.1 by adding 18 litres of semi-concentrated acid. After intake of the aluminium oxide and immediately before shearing, a sample of the suspension is taken and a d₅₀ value of d₅₀=160 nm is determined (measurement in dilution by means of dynamic light scattering). The subsequent shearing process is carried out with the rotor-stator machine at maximum shear energy and lasts for a total of 60 minutes.

[0044] After completion of the shearing process, the pH value is 4.1. After adding 2 kg of a biocide, the pH value was brought to the final pH of 3.9 with a further 6 litres of propionic acid. Once production of the dispersion is complete, the liquid volume is increased to 400 litres by adding 14 litres distilled water. The particle size set after the shearing process is d₅₀=130 nm.

[0045] Properties of the dispersion according to example 2 of the invention (VP W 520). During production of the dispersion the average particle size falls significantly as the shearing time continues (cf. FIG. 1): without shearing (0 min) the value is still d₅₀=160 nm, but after 60 minutes' shearing it falls to d₅₀=130 nm: TABLE 2 Comparison of the samples of the dispersion according to the invention immediately after production and two weeks later 0 min 15 min 27 min 30 min 45 min 60 min Shear. Shear. Shear. Shear. Shear. Shear. d₅₀ in (nm) 160 155 149 143 137 130 (day of production) d₅₀ in (nm) 170 155 153 147 139 136 (control, 2 weeks)

[0046] During production of the dispersion according to the invention, the pH value does not depend on the shearing time (cf. Table 2). The change in pH within two weeks is also the same for all samples: TABLE 3 pH-values of the samples of the dispersion according to the invention (dependency on shearing and time) 0 min 9 min 15 min 30 min 45 min 60 min Shear. Shear. Shear. Shear. Shear. Shear. pH-value 3.9 4.0 4.0 4.1 4.0 3.9 (production) pH-value 4.5 4.5 4.6 4.6 4.5 4.5 (control, 2 Weeks)

[0047] The gloss values are measured with a Byk-Gardner gloss meter using test card 2855 (black spectrum) as a basis.

[0048] The printing properties of the coating are evaluated by printing out a test image on the coating, using an HP 550 C printer and an Epson Stylus Colour 800 printer respectively and having these printed coatings evaluated by 3 independent persons on the basis of the attached scale of marks.

[0049] The colour densities are measured on the basis of the test image, which also contains full-area colours (black, magenta, cyan, yellow) using a GretagMacbeth (trademark) SpectroEye at an observation angle of 2° and a D50 light source.

EXAMPLE 3 Production of an Inkjet Coating

[0050] Educts: aluminium oxide dispersion according to example 2 polyvinyl alcohol Mowiol 40-88 (Clariant, D) polyester film

[0051] Recipe and method for the production of Inkjet coating pigments:

[0052] The aqueous polyvinyl alcohol solution (Mowiol 40-88, Clariant) with 12.14% solid content, is placed in a 400 ml beaker and a quantity of water is added to it, so that, together with the aluminium oxide dispersions according to example 2, a solid content of 18% is obtained. The relevant dispersion is slowly dropped into the polyvinyl alcohol solution using a pipette, within 5 minutes, whilst stirring at 500 rpm. Once it has been added, stirring continues for a further 30 minutes at 500 rpm to obtain a homogenous coating pigment. The coating pigments are then de-aerated using a dessicator and water-jet pump. As a control, the actual solid matter, pH value and viscosity are measured after mixing the coating pigments.

[0053] The parts in the table below are understood to mean parts by weight in relation to the solid matter. TABLE 4 Coating pigment reference values Aluminium oxide 100 parts dispersion, acc. to example 2, 15 minutes' dispersion time Aluminium oxide 100 parts dispersion, acc. to example 2, 30 minutes' dispersion time Aluminium oxide 100 parts dispersion, acc. to example 2, 60 minutes' dispersion time PVA 40-88  20 parts  20 parts  20 parts Actual solid 18.03 18.01 18.00 content of the coating pigments in % PH value 4.7 4.6 4.6 Viscosity, Brookfield in mPa s after 24 h After  5 rpm 920 824 748 stir-  10 rpm 840 716 664 ring  20 rpm 715 608 534  50 rpm 560 512 462 100 rpm 463 398 346 Comments on good good good production of coating pigments

[0054] The viscosity data obtained show that the dispersion according to example 2, and the longest dispersion time, produce the lowest viscosities. This is desirable as the solid matter in the coating pigment can still be increased without obtaining viscosities that are too high to be applied.

[0055] Method for the production of an Inkjet coating from the Inkjet coating pigment referred to above

[0056] A 100 micron thick, un-treated polyester film is coated using an Erichsen Film Applicator device with a 120 micron wet film spiral applicator. The coating pigment applied is dried using a hot air dryer.

Data for the Coatings Obtained:

[0057] Aluminium oxide 100 parts dispersion acc. To example 2, 15 minutes' dispersion time Aluminium oxide 100 parts dispersion acc. to example 2, 30 minutes' dispersion time Aluminium oxide 100 parts dispersion acc. to example 2, 60 minutes' dispersion time PVA 40-88  20 parts  20 parts  20 parts Application behaviour good good good Average application 26.1 25.9 26.2 weight in g/m² and s = 0.4 s = 0.2 s = 0.3 standard deviation* Coating properties Aluminium oxide 100 parts dispersion acc. to example 2, 15 minutes' dispersion time Aluminium oxide 100 parts dispersion acc. to example 2, 30 minutes' dispersion time Aluminium oxide 100 parts dispersion acc. to example 2, 60 minutes' dispersion time PVA 40-88  20 parts  20 parts  20 parts Adhesion, surface Good Very good Very good and smoothness of adhesion to adhesion to adhesion to the coating the film, the film, the film, homogeneous homogeneous homogeneous surface surface surface Gloss at 20°/ 24.6 26.3 28.8 standard s = 0.4 s = 0.1 s = 0.3 deviation at n = 5 Gloss at 60°/ 46.3 49.7 54.7 standard s = 0.2 s = 0.3 s = 0.3 deviation at n = 5

Discussion of the Gloss Values

[0058] It can clearly be seen from the gloss values, that the dispersion according to the invention produces higher gloss values in the Inkjet coating after 60 minutes' dispersion time, than the dispersions produced by other methods. With photo-realistic coatings, a high gloss is desirable, as already disclosed in EP 0 732 219. The gloss values are lower than those in EP 0 732 219, but this is due to the different processes for the production of the Inkjet medium and not to the coating pigment. A spiral applicator is consiously used in this test to determine the contribution of the dispersion in the Inkjet coating to the gloss. With the cast-coating process in EP 0 732 219, the gloss is primarily determined by the process itself.

[0059] Printing test (determined by 3 independent persons) Top mark: 0.75 bottom mark: 6 Aluminium Aluminium Aluminium oxide oxide oxide dispersion dispersion dispersion acc. to acc. to acc. to example 2, example 2, example 2, 15 minutes' 30 minutes' 60 minutes' dispersion dispersion dispersion Dispersion used time time time Four colour print Hewlett-Packard 550 C Colour Magenta/ 1.25 1 1 intensity Yellow/Cyan Black 1 1 1 dot Black in 1.5 1.25 1.25 sharpness colour transitions Colour to 1 1 1 colour Black to 1.25 1 1 colour Contours 1 1 1 Print 1.25 1.25 1 Halftone 1 1 1 Photo 1.25 1.25 1 quality Total 10.5 9.75 9.25 evaluation Average 1.17 1.08 1.03 marks Four colour print Epson Stylus Color 800 Colour Magenta/ 1.25 1 0.75 intensity Yellow/Cyan Black 1 1 0.75 Dot Black in 1.25 1 1 sharpness colour Transitions Colour to 1 1 1 colour Black to 1 1 1 colour Contours 1 1 1 Print 1 1 1 Halftone 1.25 1 1 Photo quality 1 1 0.75 Total 9.75 9 8.25 evaluation Average 1.08 1.00 0.92 mark

[0060] When using two different types of printer, the coating containing the dispersion with the longest dispersion time, according to example 2, produces the best results. Colour densities Aluminium Aluminium Aluminium oxide oxide oxide dispersion dispersion dispersion acc. to acc. to acc. to example 2, example 2, example 2, 15 minutes' 30 minutes' 60 minutes' Dispersion dispersion dispersion dispersion used time time time DIN HP 550 C Pap Pol Filter selected Paper white 0 0 0 Mean black 2.33 2.36 2.42 Mean yellow 1.67 1.69 1.73 Mean cyan 2.33 2.35 2.39 Mean magenta 1.39 1.42 1.46 Total 7.72 7.82 8 Epson Stylus Color 800 Pap Pol Filter selected Mean black 3.07 3.08 3.14 Mean yellow 2.15 2.17 2.21 Mean cyan 3.23 3.25 3.33 Mean magenta 2.34 2.35 2.42 Total 10.79 10.85 11.1

[0061] When examining the colour densities, it can be seen that the coating containing the dispersion according to example 2 of the invention, with the longest dispersion time, reproduces the highest colour densities. This is desirable to obtain the most photo-realistic reproduction possible.

EXAMPLE 4 Production of an Inkjet Coating

[0062] Educts: aluminium oxide dispersion according to example 2 polyvinyl alcohol Mowiol 40-88 (Clariant)

[0063] Recipe and method for the production of Inkjet coating pigments:

[0064] The aqueous polyvinyl alcohol solution (Mowiol 40-88, Clariant) containing 12.14% solid matter is placed into a 400 ml beaker and a quantity of water is added so that, together with the aluminium oxide dispersions according to example 2, a solid content of 18% is obtained. The relevant dispersion is slowly dropped into the polyvinyl alcohol solution using a pipette, within 5 minutes, whilst stirring at 500 rpm. Once it has been added, stirring continues for a further 30 minutes at 500 rpm, to obtain a homogeneous coating pigment. The coating pigments are then de-aerated using a dessicator and a water jet pump. As a control, the actual solid matter, pH value and viscosity are determined after mixing the coating pigments.

[0065] The parts in the table below are understood to mean parts by weight in relation to the solid matter.

Coating Pigment Reference Values

[0066] Aluminium oxide dispersion 100 parts acc. to example 2, 15 minutes' dispersion time Aluminium oxide dispersion 100 parts acc. to example 2, 30 minutes' dispersion time Aluminium oxide dispersion 100 parts acc. to example 2, 60 minutes' dispersion time PVA 40-88  18 parts  18 parts  18 parts Actual solid content of 18.01 18.04 18.02 the coating pigment in % pH-value 4.7 4.6 4.6 Viscosity, Brookfield in mPa s after 24 h  5 rpm 912 813 748  10 rpm 846 697 664  20 rpm 702 589 534  50 rpm 535 496 462 100 rpm 443 378 346 Comments on production of good good good coating pigments

[0067] The viscosity data obtained show that the lowest viscosities are obtained with the dispersion according to example 2 and the longest dispersion time. This is desirable, as the solid matter in the coating pigment can be increased still further without obtaining viscosities that are too high for application.

[0068] Method for the production of an Inkjet coating from the Inkjet coating pigment referred to above.

[0069] Matt-coated 110 g/M² Inkjet paper (Zweckform no. 2576) is coated using an Erichsen Film Applicator device with a 60 micron wet film spiral applicator. The coating pigment applied is dried with a hot air dryer.

[0070] The coatings are then calendered three times at 10 bar pressure and 50° C. with a Gradek Oy™ laboratory calender.

Data for the Coatings Obtained

[0071] Aluminium oxide 100 parts dispersion acc. to example 2, 15 minutes' dispersion time Aluminium oxide 100 parts dispersion acc. to example 2, 30 minutes' dispersion time Aluminium oxide 100 parts dispersion acc. to example 2, 60 minutes' dispersion time PVA 40-88  18 parts  18 parts  18 parts Application behaviour good good good Average coating weight 12.0 11.9 12.1 in g/m² and standard s = 0.4 s = 0.2 s = 0.3 deviation*

[0072] Coating properties Aluminium oxide 100 parts dispersion acc. to example 2, 15 minutes' dispersion time Aluminium oxide 100 parts dispersion acc. to example 2, 30 minutes' dispersion time Aluminium oxide 100 parts dispersion acc. to example 2, 60 minutes' dispersion time PVA 40-88  18 parts  18 parts  18 parts Adhesion, surface and Very good Very good Very good smoothness of the adhesion to adhesion to adhesion to coating the paper, the paper, the paper, homogeneous homogeneous homogeneous surface surface surface Gloss at 20°/  5.4  5.9  6.7 standard deviation at s = 0.4 s = 0.1 s = 0.3 n = 5 Gloss at 60°/ 24.6 25.7 26.2 standard deviation at s = 0.2 s = 0.3 s = 0.3 n = 5

Discussion of the Gloss Values

[0073] The gloss values clearly show that, after 60 minutes' dispersion time, the dispersion according to the invention produces higher gloss values in the Inkjet coating than dispersions produced by other methods. For photo-realistic coatings, a high gloss is desirable as already disclosed in EP 0 732 219. The gloss values are lower than those in EP 0 732 219, but this is due to the different processes for the production of the Inkjet medium and not to the coating pigment. A spiral applicator is consciously used in this test, to determine the contribution of the dispersion in the Inkjet coating to the gloss. With the cast-coating process used in EP 0 732 219, the gloss is primarily determined by the process it self.

[0074] Printing test (determined by 3 independent persons) Top mark: 0.75 Bottom mark: 6 Aluminium Aluminium Aluminium oxide oxide oxide dispersion dispersion dispersion acc. to acc. to acc. to example 2, example 2, example 2, 15 minutes' 30 minutes' 60 minutes' dispersion dispersion dispersion Dispersion used time time time Four colour print Hewlett-Packard 550 C Colour Magenta/ 1.25 1.25 1 intensity Yellow/Cyan Black 1.25 1.25 1 Dot Black in 1.5 1.25 1.25 sharpness colour Transitions Colour to 1 1 1 colour Black to 1 1 1 colour Contours 1 1 1 Print 1.25 1.25 1 Halftone 1.25 1 1 Photo 1.5 1.25 1.25 quality Total 11 10.25 9.5 evaluation Average 1.22 1.08 1.06 mark Four colour print Epson Stylus Color 800 Colour Magenta/ 1.5 1.25 1 intensity Yellow/Cyan Black 1 1 1 Dot Black in 1.5 1.25 1.25 sharpness colour Transitions Colour to 1.25 1.25 1 colour Black to 1 1 1 colour Contours 1 1 1 Print 1 1 1 Halftone 1.25 1 1 Photo 1.5 1.25 1.25 quality Total 11 10 9.5 evaluation Average 1.22 1.11 1.06 mark

[0075] When using two different types of printer, the coating containing the dispersion with the longest dispersion time according to example 2, produces the best results. Colour densities Aluminium Aluminium Aluminium oxide oxide oxide dispersion dispersion dispersion acc. to acc. to acc. to example 2, example 2, example 2, 15 minutes' 30 minutes' 60 minutes' Dispersion dispersion dispersion dispersion used time time time DIN HP 550 C Pap Pol Filter selected Paper white 0 0 0 Mean black 2.05 2.10 2.13 Mean yellow 1.49 1.54 1.58 Mean cyan 2.04 2.07 2.11 Mean magenta 1.28 1.32 1.35 Total 6.86 7.03 7.17 DIN Epson Stylus Color 800 Pap Pol Filter selected Mean black 2.89 2.96 2.98 Mean yellow 1.92 1.97 2.04 Mean cyan 2.96 3.03 3.09 Mean magenta 2.13 2.25 2.31 Total 9.9 10.21 10.42

[0076] When examining the colour densities, it can be seen that the coating containing the dispersion according to example 2 of the invention with the longest dispersion time reproduces the highest colour densities. This is desirable to obtain the most photo-realistic reporoduction possible.

[0077] The dispersion according to the invention is eminently suitable for the production of Inkjet coating pigments and their further processing to produce high-gloss printing media, as can be seen from the examples given. The Inkjet media produced in this way have a particularly good print and gloss quality. 

1. Dispersion of aluminium oxide, characterised in that it contains high-surface-area, pyrogenically-produced aluminium oxide, which has a BET specific surface area of more than 115 m²/g and a Sears number of more than 8 ml/2 g.
 2. Dispersions of aluminium oxide according to claim 1, characterised in that they also contain, in addition to aluminium oxide, inorganic acids (such as phosphoric acid, nitric acid, sulfuric acid, . . . ), organic acids (such as formic acid, acetic acid, propionic acid, oxalic acid, malonic acid . . . ), inorganic or organic bases (such as potassium hydroxide, organic amines, . . . ), salts (such as sodium chloride, potassium formate, calcium nitrate), buffer systems (such as for example potassium dihydrogenphosphate/phosphoric acid buffer, acetic acid/sodium acetate buffer, . . . ), ionic or non-ionic surfactants, polyelctrolytes, polymers or other dispersing agents or stabilisers and also biocides.
 3. Process for the production of aluminium oxide dispersions according to claim 1, characterised in that a high-surface-area, pyrogenically-produced aluminium oxide, which has a BET specific surface area of more than 115 m²/g and a Sears number of more than 8 ml/2 g, is mixed with water, a pH value of 2 to 11 (preferably 3 to 8) is set, and the mixture is dispersed by the introduction of controlled shearing forces.
 4. Process for the production of aluminium oxide dispersions according to claim 3, characterised in that a high-surface-area, pyrogenically-produced aluminium oxide, which has a BET specific surface area of more than 115 m²/g and a Sears number of more than 8 ml/2 g is mixed with water and additives, a pH value of 2 to 11 (preferably 3 to 8) is set, and the mixture is dispersed by the introduction of controlled shearing forces.
 5. Process for the production of aluminium oxide dispersions according to claim 3, characterised in that shearing equipment such as rotor-stator machines (batch- or continuous in-line machines), ball mills, pearl mills, agitated ball mills or high-energy shearing processes (in which dispersion is carried out under high pressure at pressures of more than 1000 bar) is used to disperse the high-surface-area, pyrogenically-produced aluminium oxide, and the dispersions obtained have a mean particle size distribution of d₅₀<150 nm (d₅₀ less than 150 nm; measured by dynamic light scattering).
 6. Coatings according to claim 1, wherein the coating has an aluminium oxide-binder weight ratio of 8:1 to 1:2.
 7. Recording media of aluminium oxide dispersions according to claim 1, characterised in that they consist of substrates with glossy coatings, in which the coatings contain VP Alu 130 dispersions and also dispersion additives (such as acids, bases, surfactants, polymers, polyelectrolytes etc.) and a binder system, the aluminium oxide particles from which the coatings are built up originating from a dispersion according to the invention, with a mean particle size of d₅₀<200 nm (d₅₀ less than 200 nm; measured by dynamic light scattering).
 8. Use of the coatings according to claim 6 for the production of Inkjet media, digital imaging products and other printing media.
 9. Use of the aluminium oxide dispersion in coatings for materials such as paper, metals, glass, ceramics, plastics, polymers, rubbers and textiles.
 10. Use of the aluminium oxide dispersion according to claim 1 for polishing and cleaning metals, semi-conductor elements in the electronics industry, glass, ceramics and other hard materials.
 11. Coatings according to claim 6, produced from coating pigments, which have a solids content of 15%, preferably 18% or more.
 12. Coatings according to claim 6, wherein the substrate has a surface weight of 80 to 250 g/m².
 13. Coatings according to claim 6, wherein the substrate is paper.
 14. Coatings according to claim 6, wherein the substrate has a Cobb₁₀ value of 0 to
 60. 15. Coatings according to claim 6, wherein a water-impermeable sealing layer has been applied to the substrate.
 16. Coatings according to claim 6, wherein the substrate is a polymer film.
 17. Use of the coatings according to claim 6 for the production of Inkjet media (such as paper, films, textiles, . . . ), digital imaging products and other printing media.
 18. Use of the aluminium oxide dispersions according to claim 1 for the coating of fluorescent tubes, lightbulbs or other light sources. 